The present invention relates to offshore platforms, and specifically to offshore platforms designed for dry tree applications. More particularly, the present invention relates to a new production and/or drilling riser system used in deep draft semi-submersible platforms.
Conventional dry tree offshore platforms are low heave floating platforms, such as spars, TLPs (Tension Leg Platforms), and deep draft semi-submersible platforms. These platforms are able to support a plurality of vertical production and/or drilling risers. These platforms may comprise a well deck, where the surface trees (arranged on top of the riser) will be located, and a production deck where all the crude oil will be manifolded and sent to a processing facility to separate water, oil and gas. In conventional dry tree offshore platforms, vertical risers running from the well head to the well deck are supported by a tensioning apparatus. These vertical risers are called Top Tensioned Risers (TTRs).
One prior art TTR design uses active hydraulic tensioners to independently support the risers. Each riser extends vertically from the wellhead to the well deck of the offshore platform. The riser is supported by active hydraulic cylinders connected to the well deck of the offshore platform, allowing the platform to move up and down relative to the risers and thus partially isolating the risers from the heave motions of the hull. A surface tree is connected on top of the riser, and a high pressure flexible jumper connects the surface tree to the production deck. As tension and stroke requirements increase, these active tensioners become prohibitively expensive. Furthermore, the loads have to be supported by the offshore platform.
A second prior art design uses passive buoyancy cans to independently support the risers. Each riser extends vertically from the wellhead to the well deck of the offshore platform. The riser passes from the wellhead through the keel of the floating platform into a stem pipe, on which buoyancy cans are attached. This stem pipe extends above the buoyancy cans and supports the platform to which the riser and the surface tree are attached. A high pressure flexible jumper connects the surface tree to the production deck. Because the risers are independently supported by the buoyancy cans (relative to the hull), the hull is able to move up and down relative to the risers, and thus the risers are isolated from the heave motions of the offshore platform. The buoyancy cans need to provide enough buoyancy to support the required top tension in the risers, the weight of the can and the stem pipe, and the weight of the surface tree. With increased depth, the buoyancy required to support the riser system will also increase, thereby requiring larger buoyancy cans. Consequently the deck space required to accommodate all the risers will increase. Designing and manufacturing individual buoyancy cans for each riser is also costly.
Offshore environmental conditions are often harsh. Actions of wind, waves and currents on an offshore structure can have severe effects, especially in the layer of the sea between the surface and a depth of about 150-300 ft. (about 45 m to about 90 m) which is called the “splash zone”. These actions attenuate with the water depth. In deep draft semi-submersible platforms, the vertical risers are subjected to the effects of high waves and current forces near the surface, which puts strain on the risers and can lead to VIV (Vortex Induced Vibrations). Consequently, in both of the aforementioned designs, each riser must be provided with strakes to prevent or minimize VIV, thereby increasing manufacturing costs.
A third prior art design, exemplified by U.S. Pat. Nos. 5,439,321 and 4,913,238, proposes to connect all the TTRs to a single (independent from the work platform) buoyancy apparatus in order to create a kind of small well deck TLP (Tension Leg Platform) to be received in a conventional semi-submersible platform. The small well deck TLP will be anchored with tendons connected to the outer periphery of the buoyancy apparatus. The well deck TLP is not dependent from the floating platform. In the apparatus disclosed in U.S. Pat. No. 5,439,321 the well deck TLP is connected to the floating platform through a cross springs mooring system, and in the apparatus disclosed in U.S. Pat. No. 4,913,238, through centralizer dollies arranged at the bottom of the floating platform. This device restrains the TLP partially horizontally; however the TLP is still able to rotate relative to the platform. The well deck TLP through this anchoring system has very good motion characteristics; however the conventional semi-submersible platform has large motions which will be transmitted to the well deck TLP, and the tendon and riser system must be designed to withstand these horizontal and pitch motions as well as large impact loads between the two floating vessels. Furthermore, as the conventional semi-submersible platform undergoes large motions, long, flexible jumpers to carry crude oil from the well deck TLP to the production deck on the semi-submersible platform are required to absorb the large relative motions between the two vessels. Finally, the vertical risers are connected only in the upper part of the single buoyancy apparatus. Nothing is proposed for horizontal restraint of the motion of the risers within the buoy.